The present invention relates to a heating resistance and to the means for securing it to the insulating support base of a radiant heater specially adapted for a glass ceramic hob.
Electric radiant heaters for glass ceramic hobs in which the heating resistance is made from a thin flat strip of high working temperature alloy, as described in EP-750444-A (U.S. Pat. No. 5,834,740), are already known. The resistance comprises a thin strip of the same width over its whole length, between 1.5 mm-7 mm, variable in accordance with the power, which is first shaped in undulating form and then set in place securely on the horizontal insulating base of the heater, supporting its edge. The resistance strip has its own integral fixing tabs protruding from one of the edges of the strip and spaced at a regular distance from one another, at considerable intervals of resistance length. The insulating base is made of a microporous heat-insulating material and the fixing tabs are inserted on it, so that the resistance strip is left in a vertical position.
The fixing tabs integral with the strip give rise to an irregular conductor section along the resistance, which produces differences in temperature that accelerate its thermal fatigue. The manufacture of a resistance strip with integrated tabs calls for a process of stamping of two simultaneous resistance strips from a double-width alloy, which must be high precision in order to achieve the same conductor section of the resistance strip over its whole length, as its power rating is determined afterwards by means of the length of strip only. Another drawback of the solutions with integral fixing tabs is that they require a change of die for stamping the resistance strips when a different distance between tabs is sought.
Heat dissipation by way of the fixing tabs has to be the minimum possible so as not to generate a cold area around the tab that alters the overall working temperature and produces thermal stress. A resistance strip for a radiant heater is very thin, with a thickness of 0.04 mm-0.15 mm, so the integral tab is also very thin. The integral tabs must be of low height to facilitate the stamping of the strip with dies, while at the same time of large area to achieve lasting anchorage of the resistance and to prevent its bending during insertion. The short fixing tabs call for a large number of tabs per section of length, as the interval between two successive tabs is a decisive factor for the resistance to remain in place on the insulating base throughout the life of the cooking plate.
Furthermore, in the solution shown in the afore-mentioned prior art document, the tab area is small, but the tab has to be curved in the form of a blade in order to improve anchorage, while the strip must necessarily be bent along the line of the tab during its undulating shaping. The simultaneous bending of the resistance strip and the tab adds a difficulty to the manufacture of the resistance.
The object of the invention is a flat electrical heating resistance for a radiant heater of a glass ceramic hob cooking plate, provided with a series of retaining lugs for its installation in a vertical position on the porous insulating base of the radiant heater, as defined in claim 1.
The present invention provides a system for fixing the heating resistance different from that of the solution described above in the prior art. The resistance strip has retaining lugs, welded on one of its sides at well spaced out intervals along the resistance in order to avoid cold areas on the resistance, while it is also sturdy and has a lug projecting from the lower edge that is relatively high but of small section, chosen in each case in accordance with the strip width.
The resistance strip is formed by cutting it out of an alloy band or ribbon of larger width in order to obtain several strips at the same time, so that the whole width of the band is utilised with no wastage of material. The cutting process is simple in comparison with the stamping of the band to obtain two strips with integrated tabs, as it is done in the prior art, and furthermore, compared with the integral tab strips, a resistance conductor section is obtained that is the same over its whole length. The thickness of the lug, greater than that of the resistance strip, may be chosen in each case so that the lug is resistant to bending regardless of the thickness of the resistance strip.
Through not needing stamping dies for the lugs, the heating resistance according to the present invention also offers the advantage of flexibility in the range of heating resistance power ratings. The lugs are welded onto the resistance strip prior to its undulation bending on an automatic machine that synchronises the positioning of the strip and lugs under the welding electrode. Thus, the length of the retaining lugs is the only variable in accordance with the radiant heater power, without the need to change, the strip or lug feed sequence on the welding machine.